The traditional three piece railway freight car truck consists of one bolster and two side frames. The side frames are supported at their ends by the wheelsets. The bolster which carries the car body extends centrally through the side frames. The bolster is supported on suspension springs with damping friction shoes located in the side frames that support the bolster. The suspension contains load springs that support the bolster and control springs that support the friction shoes. The friction shoes include angled surfaces that bear against the bolster in pockets that have mating angled surfaces. The result of the spring force acting on friction shoe against the angled support of the bolster is a wedge force acting on the side frame. Damping is the result of the wedge force on the friction shoe flat surface sliding against and along the flat surface of the side frame. The resulting wedge force and friction between the friction shoe flat surface and the side frame flat surface creates sliding force resistant to movement. There are generally two types of friction damping systems in use constant and variable. In the constant damped system, the springs creating the force against the wedge are not load bearing and friction force is the same regardless of load on the truck centerplate so the shoe sliding force resistance is constant. In the variable damped system, the springs applying the force to the wedges are also load bearing springs and apply more force to the wedge when under load than when the freight car is empty so the shoe sliding force resistance increases as the springs are compressed.
In some variable damped systems the wedge springs have little or no compression and there is insufficient shoe sliding force resistance when the freight car is empty. Under these conditions, vertical track irregularities cause the springs to be compressed and store energy and when the energy is released, without sufficient sliding force resistance, the energy and inertia cause the bolster and freight car body to travel vertically through the neutral point and the bolster may rise up off the load springs. Sometimes excessive vertical impact can occur between truck components possibly damaging them; further, springs can become dislocated out of their seat locations.
In addition to insufficient friction damping, there are other situations such as extreme vertical track irregularities or loading and unloading operations, which can cause the bolster to move vertically by an excessive amount and thereby impacting the side frame, or brake equipment, or causing displacement of springs.
The present invention relates to limiting the vertical travel of the bolster and absorbing the energy before damage can occur to truck components.